docs: update venv activation for windows (#1836)

## Summary

Follow on to PR #1811 as part of
issue #1750. This change updates
the windows venv activation to work for both Powershell and Command
Prompt instead of just Powershell. This also aligns with what `uv venv`
displays.

README.md

@@ -66,7 +66,7 @@ To activate the virtual environment:
 source .venv/bin/activate
 
 # On Windows.
-.\.venv\Scripts\activate.ps1
+.venv\Scripts\activate
 ```
 
 To install a package into the virtual environment: